Thirteen patients with hypertrophic cardiomyopathy have been studied with incremental arterial and ventricular pacing during diagnostic cardiac catheterization. The data suggest that 1) the detremental clinical effect of atrial or ventricular tachyarrhythmias is primarily mediated by an elevation in filling pressure and a decrease in cardiac output, rather than an augmentation of outflow obstruction, and 2) at slow rates (less than or equal to 110 bpm) atrial pacing is hemodynamically advantageous.